Field of the Invention
The present invention relates to a hydroelastic joint for assembling two pieces of a structure and for damping vibrations transmitted between each other. More precisely, the invention relates to a joint of the type comprising an external reinforcement and an internal reinforcement, each having a longitudinal axis, which are disposed one around the other and intended to be fixed respectively to one and to the other of the pieces to be assembled, and an assembly forming a hydroelastic spring disposed between said reinforcements in order to allow a relative transverse displacement between said reinforcements, said assembly comprising a first elastically deformable element shaped in order to delimit between said reinforcements at least one sealed volume containing damping fluid.
These joints are likewise designated by the terms support, strut, sleeve or “bushing”. They have two main functions: to offer degrees of freedom between the pieces which they assemble and to damp, to a greater or lesser extent according to the intended application, the transmission of vibrations between one and the other of these pieces.
In the field of automotive vehicle construction, these joints are used in particular for the assembly and damping of ground contact members, such as axles or suspension triangles of wheel and axle assemblies, relative to the main structure or body of the vehicle.
In this case, it is the displacement modes in the longitudinal direction of the vehicle, at which the damping is particularly aimed, such as the backward movement of a wheel on contact with an obstacle. Known vibration sources at the level of the ground contact members of a vehicle are also the unbalance of the wheels, the non-uniformity of the tyres when running, faults in the brake discs and devices for assisting braking. The vibrations of the ground contact members are generally characterised by relatively low resonance frequencies, for example between 15 and 20 Hz, and relatively high amplitudes, for example of the order of one millimeter or more, such that they are perceptible by the occupants of the vehicle if incorrectly damped.
For example, it is known to fix, by means of two joints of this type, a deformable axle, termed as H, to the body of a vehicle. These joints ensure in particular the maintenance of the axle during cornering. FIG. 9 illustrates such an assembly.
With reference to FIG. 9, the axle 51 is a rear axle of the deformable type. It comprises a transverse beam 52, which is rigid in flexure and bears on its two ends a respective longitudinal arm 53a, 53b. Each longitudinal arm 53a, 53b bears in turn a respective wheel support, on which a respective wheel 54a, 54b is mounted at a first end, termed rear end, and a joint 55a, 55b at the other end, termed front end. Each joint is fixed to the longitudinal arm by one of its reinforcements, internal and external, and fixed to the body of the vehicle, which is not represented here, by the other reinforcement.
During cornering of the vehicle, on the one hand the lateral inertia force F1 being exerted on the body and, on the other hand, the lateral frictional force F2 being exerted on the wheels, cause, between the axle 51 and the body, a displacement which can be broken down into a translation according to the transverse direction of the vehicle and a rotation about a vertical axis. This joint displacement and deformation of the axle can cause the vehicle to oversteer: the back wheels, having a very significant steering power, increase the steering lock which increases in turn the inertia forces etc.
In order to correct this fault it has been considered to use self-steering elastic joints, the rigidity of which in the various directions, in particular axial and radial, are controlled and orientated relative to a reference system connected to the vehicle in order to bring about a displacement of the opposite axle. FIG. 9 illustrates this type of assembly, the longitudinal axes of the joints 55a and 55b being orientated in a horizontal plane in order to form an angle α relative to the transverse direction of the vehicle Y, defined by the wheel supports 54a and 54b. The arrows F3 represent the resultant stress sustained by each joint.
In order to obtain a good level of performance in this type of assembly, as far as the guidance of the axle is concerned, it is necessary that the ratio of the radial rigidity to the axial rigidity of the joints is as high as possible, i.e. the behaviour of each joint is as close as possible to the behaviour of an axial slide. The theoretical center of rotation C of the axle, at the intersection of the action lines of the forces F3, is therefore displaced proportionally towards the actual center of rotation of the vehicle. A high value can be obtained for this ratio, in the known manner between approximately 1 and 4, by designing the assembly as a hydroelastic spring in an appropriate manner.
However, it is not desirable to replace the hydroelastic joints with a real slide because then one would lose any damping of the vibrations, which is hardly permissible from the point of view of the comfort of the vehicle.
Thus, there is always a disadvantage in that the joints of the above-mentioned type are subject to impairment or premature aging when they are subjected to a stress which tends to vary the angle formed by the respective longitudinal axes of the internal reinforcement and the external reinforcement, i.e. to induce a relative tilting movement of said longitudinal axes about a transverse direction perpendicular to the two axes. Such a deformation of the joint is also termed conical deformation.
It is clearly apparent that the joints 55a and 55b are subjected to a conical deformation during a vertical deflection of the wheels 54a and 54b upon contact with non-planar terrain. Another disadvantage results therefrom in that the necessity for limiting the conical deformation within permissible limits, for example with a maximum tilting less than 10°, imposes a constraint upon the orientation which it is possible to give to the joint, which complicates the design of the ground contact members and restricts their performances.
Stresses tending to vary the angle formed by the respective longitudinal axes of the internal reinforcement and the external reinforcement can also appear in numerous other applications of hydroelastic joints.
The reason for such impairment is that the conical deformation stresses the first elastic element by deformation at the level of its ends, for example in tension/compression, shearing, flexion, torsion or any combination of stresses. However, because of the fact that it delimits the sealed volume containing the damping fluid, the first elastic element has end walls which have a transverse dimension and a longitudinal spacing imposed by the volume of fluid to be contained, which determines the effectiveness of the hydroelastic operation. As a result, the amplitude of the transverse deformation at the level of the end walls when the joint is subjected to a conical deformation is able to make the end walls operate in a manner prejudicial to the longevity of the elastomer, for example by transverse tension/compression. One might consider modifying the dimensions, for example transverse, of the walls, but the necessity of preserving the desired rigidity values in all directions restricts this possibility.
Likewise, the transverse compressive pretensioning which it is possible to apply to the first elastic element at the level of its ends is reduced as a result of the fact that such pretensioning tends, either to crush the end walls and to reduce the volume available for the damping fluid if the compression takes place before filling of the volume, or to axially distend the end walls if the compression takes place after filling of the volume. Furthermore, such pretensioning presents great difficulties in implementation.
The document FR 2 784 152 describes a joint of the above-mentioned type which comprises furthermore a second elastically deformable element of a similar design to the first and disposed between the first elastically deformed element deformable element and one of the reinforcements, external and internal, in order to form a second hydroelastic spring mounted in series with the first between the two reinforcements. However, this device does not present a satisfactory solution to the above-mentioned disadvantage since it substantially doubles the spatial requirement and the cost of the joint without preventing, for each of the two elastic elements, premature aging at the level of the end walls under conical deformation.